1. Field of the invention
The present invention relates to a super twisted nematic type liquid crystal display device used as the display unit for word processors, personal computers and other office automation equipment.
2. Description of the Related Art
Generally a supertwisted nematic (STN) liquid crystal display device is colored in yellow-green or blue, but the color is corrected by using optical compensation plates to obtain a bright and sharp black and white display. This improves the display quality and makes the device applicable for use as the display unit of word processors apparatuses, personal computers and other office automation equipment.
As an STN type liquid crystal display device using optical compensation plates, there is a two-layer STN type liquid crystal display device using two panels stacked on each other, where the coloration taking place in the first panel used in display driving is compensated for by the second layer (optical compensation panel) to provide a colorless display. This two-layer configuration requires two liquid crystal panels, as compared with the single-layer STN liquid crystal display device, resulting in increased thickness of the display device and consequently increasing the weight.
To solve this problem, a thin and light-weight STN type liquid crystal display device has been developed by using a positive phase difference plate made of a uniaxially stretched polymer film with coefficient K=1 as the optical compensation plate. The coefficient K is a value representing the rate of retardation change of light viewed through the film as the inclination of the viewing direction, or "elevation angle," changes. In a uniaxially stretched polymer film exhibiting anisotropic optical properties, the coefficient K is defined by the mutually orthogonal three-dimensional refractive indices of the film (in two directions along the film plane and in a third direction normal to the film plane). In the description which follows, the elevation angle is measured from a direction normal to the plane of the film. Range of the coefficient is 0.ltoreq.K.ltoreq.1 for a positive phase difference plate and -1.ltoreq.K.ltoreq.0 for a negative phase difference plate. The phase difference plate is, however, in general manufactured by stretching a polymer film, and therefore has optical properties different between the slow/axis direction and the fast/axis direction. Consequently, there is a problem, as compared with the two-layer STN type liquid crystal display device, that the color change due to the azimuth or elevation angle is greater in the STN liquid crystal display device of phase difference plate type, that is, the viewing angle is narrower.
The reason for using a uniaxially stretched polymer film as a phase difference plate is the optical anisotropy of the film. That is, a polymer film has different refractive indices between the fast axis direction and the slow axis direction (birefringence). The retardation (.DELTA.n.times.d), given by the product of this refractive anisotropy .DELTA.n and the film thickness d, is a physical quantity which gives the phase difference of light taking place as it passes through the film, and this value changes with the elevation angle differently between the slow axis direction and the fast axis direction.
In a phase difference plate made of polyvinyl alcohol, for example, the retardation decreases in the slow axis direction and increases in the fast axis direction as the elevation angle increases. As a result, when combined with a liquid crystal display panel, the difference in retardation between the phase difference plate and the liquid crystal display panel increases as the elevation angle increases, even if the optical compensation relation is perfect in the normal direction, and consequently the optical compensation relation collapses. In other words, a color change occurs and the display contrast decreases, resulting in narrower viewing angle.